There is no simple answer to whether the weight of the premature baby on its
actual "due date" would be similar to what it would have weighed at birth if
it had been born on its due date.

First, remember that a due date is just a "guesstimate" and that actual
normal human pregnancies can last a variety of lengths of time.  Children
may be born two weeks before or two weeks after their due date and yet be
exactly "on time" for them and their mother.  The due date is not magical or
mystical, nor does it represent the day the baby would actually have been
born if not premature.  All three of my children were conceived within three
days of my last menstrual period ending (very early in the cycle).  The
first was born two weeks LATE according to the calculated "due date", the
second was born one month EARLY according to the calculated "due date" (but
was obviously NOT premature), and the third was born two days LATE according
to the calculated "due date."

Second, babies can be premature for many different reasons, including
problems with the baby, including growth problems, as well as problems with
the mother.  A baby who is premature for one reason may grow differently
once outside of the womb than a baby who is premature for a different
reason.  It probably matters whether the prematurity is because of a problem
on the baby's part, the mother's part, the placenta's part (actually part of
the baby), or some combination, or something else.

Third, the fetus may reach the limit of the uterus' capacity for growth well
before the due date.  In such cases, the fetus doesn't grow much at all
during the last few weeks in utero due to uterine size constraints.  After
birth, the baby will grow in leaps and bounds to catch up to its genetic
track or channel.  Thus, a baby born three weeks premature, but who had
reached the limits of uterine growth and who weighs, say 6 lbs. 6 oz., may
have weighed exactly the same if she had been born on her due date.  If she
is born three weeks early, since she is freed of the uterine growth
constraints early, and can grow rapidly, she probably will weigh more on her
actual due date than she would have if she had stayed inside.

Fourth, remember that birth weights MAY BE affected by IV fluids in the
mother, and certainly growth after birth depends on what and how much is put
into the baby (formula, sugar water, breast milk, etc.).

Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University